In recent years, in accordance with an increase in the amount of data transmitted over a network, a higher transmission capacity has been increasingly demanded. In order to achieve an increase in the transmission capacity, an optical modulation device including an optical modulator that is capable of modulating light at high speed is employed. Such an optical modulation device includes an optical modulator, such as an LiNbO3 (LN) modulator that modulates, using a received electric signal, light emitted from a light source, and outputs, as a light signal, the light modulated by the optical modulator.
In optical modulators, for example, due to a variation in the temperature, a variation with time, and the like, periodic characteristics relating to the relationship between the magnitude of the bias voltage of an electric signal input to an optical modulator and the intensity of a light signal output from the optical modulator (hereinafter, referred to as “period characteristics”) vary. There is a problem in that a variation in the period characteristics causes the bias voltage of an electric signal input to an optical modulator to be deviated from an optimal value.
Under such circumstances, a technique for automatically controlling the bias voltage of an electric signal to be input to an optical modulator is suggested. FIG. 8 illustrates the configuration of an optical modulation device that automatically controls a bias voltage. The optical modulation device includes a pilot signal generating circuit 11, a data signal generating circuit 12, a pilot signal superimposing unit 13, a laser diode (LD) 14, and an LN modulator 15. The optical modulation device also includes digital analog converters (DACs) 16a and 16b, an optical coupler 17, a photo detector (PD) 18, a current/voltage converter (I/V converter) 19, a band pass filter (BPF) 20, and an analog digital converter (ADC) 21. The optical modulation device also includes a delay circuit 22, a phase comparator 23, a bias voltage controller 24, and a bias voltage applying circuit 25.
A pilot signal having a sinusoidal wave shape and generated by the pilot signal generating circuit 11 is converted, via the DAC 16a, into an analog signal. A data signal generated by the data signal generating circuit 12 is converted, via the DAC 16b, into an analog signal.
A pilot signal is a low-frequency signal having a frequency lower than the frequency of a data signal. The pilot signal from the DAC 16a is superimposed, by the pilot signal superimposing unit 13, on the data signal from the DAC 16b. An input signal that is obtained by superimposing the pilot signal on the data signal is input via the bias voltage applying circuit 25 to the LN modulator 15. The LN modulator 15 modulates, using the input signal, light generated by the LD 14, and outputs the modulated light as a light signal.
The light signal output from the LN modulator 15 is split by the optical coupler 17. The split light is converted, by the PD 18, into a current signal, and then is converted, by the I/V converter 19, into a voltage signal. Then, the voltage signal output from the I/V converter 19 passes through the BPF 20, and a low-frequency component, which is a frequency component having the same frequency as the frequency of the pilot signal is extracted. The low-frequency component extracted by the BPF 20 is converted, by the ADC 21, into a digital signal, and then is input to the phase comparator 23. The low-frequency component extracted by the BPF 20 exhibits a sinusoidal wave shape corresponding to the sinusoidal wave shape of the pilot signal.
The pilot signal generated by the pilot signal generating circuit 11 is converted, via the DAC 16a, into an analog signal. A delay time provided by the delay circuit 22 is added to the analog pilot signal, and the pilot signal including the delay time added thereto is input to the phase comparator 23. The delay time added to the pilot signal corresponds to the time to be taken for processing by the LN modulator 15, the optical coupler 17, the PD 18, the I/V converter 19, the BPF 20, and the ADC 21. In other words, the delay time added to the pilot signal corresponds to the time for compensating for a delay of the low-frequency component extracted by the BPF 20 with respect to the pilot signal.
The phase comparator 23 compares the phase of the pilot signal that has been subjected to the processing of the delay circuit 22 with the phase of the low-frequency component extracted by the BPF 20. The bias voltage controller 24 controls, with reference to the result of the comparison between the phases by the phase comparator 23, the bias voltage to be applied to the data signal by the bias voltage applying circuit 25. That is, the bias voltage controller 24 controls the bias voltage of the input signal to be input to the LN modulator 15 to minimize the low-frequency component extracted by the BPF 20.
Accordingly, by comparing the phase of a pilot signal to be superimposed on a data signal with the phase of a low-frequency component extracted from a light signal and controlling a bias voltage in accordance with the result of the comparison between the phases, the bias voltage may be automatically adjusted to an optimal value so as to follow changes in the period characteristics.
An example of related art is described in Japanese Laid-open Patent Publication No. 2004-056187.
However, the related art for controlling a bias voltage in accordance with the result of the comparison between phases does not take into account control for the bias voltage of a signal to be input to an optical modulator with high speed and high efficiency.
Namely, in the related art, it is presumed that a delay time for compensating for a delay of the low-frequency component extracted from a light signal is added to a pilot signal whose phase is to be compared with the phase of the low-frequency component. Thus, an optical modulation device for which a delay time is not set is not capable of comparing the phase of a pilot signal with the phase of a low-frequency component, and controlling the bias voltage in accordance with the result of the comparison between the phases does not start. As a result, in the related art, high-speed and high-efficiency control for the bias voltage of a signal to be input to an optical modulator may be inhibited.